In recent years, desalting seawater or brackish water using a composite semipermeable membrane has been tried, and now, it has been put into practical use in water plants all over the world. Composite semipermeable membranes each comprise a microporous support membrane coated with a separating functional layer, and it is known that when the separating functional layer is formed from a crosslinked aromatic polyamide, there is an advantage that it is high in stiffness because of containing benzene rings and a membrane can be produced readily by interfacial polycondensation of an aqueous aromatic polyfunctional amine solution and an organic solvent solution of an aromatic polyfunctional acid halide, and a high salt removal ratio and a high permeation flow are exhibited.
However, when a composite semipermeable membrane produced by forming a separating functional polyamide layer on a microporous support membrane is dried, it develops quality changes, such as decrease in water permeability or solute removal ratio, and therefore it is necessary to keep moisture during the storage thereof. Although a method in which interfacial polymerization of an aqueous solution of a polyfunctional amine and an organic solvent solution of a polyfunctional acid halide is performed in the presence of a macromolecular wetting agent having two or more ionic portions (Patent document 1), for example, is disclosed as a method of preventing quality changes caused by drying, a composite semipermeable membrane obtained by this method undesirably tends to become lower in solute removal ratio than a membrane obtained in the absence of such a macromolecular wetting agent. In addition, there is disclosed a method in which a composite semipermeable membrane obtained by interfacial polymerization of an aqueous solution of a polyfunctional amine and an organic solvent solution of a polyfunctional acid halide is immersed in a solution of a saccharide having a molecular weight of 1000 or less (Patent document 2). However, it is necessary for this method to use a saccharide solution with a high concentration of 5% by weight or more, and therefore there is a problem that an economic burden and a load for waste liquid treatment increase.